This invention relates to printed circuits and their method of manufacture and, more specificially, to a printed circuit comprising multiple layers and rigid and flexible portions comprising, a sheet of flexible substrate material extending over the entirety of the rigid and flexible portions; paths of conductive material carried by at least one side of the sheet of flexible substrate material; a sheet of flexible over-layer material extending over at least the entirety of all the flexible portions; a flexible adhesive material adhesively attaching the sheet of flexible over-layer material to the entirety of all the flexible portions; sheets of a rigid substrate material extending over the entirety of all the rigid portions; and, a rigid adhesive material adhesively attaching the sheets of a rigid substrate material to the entirety of all the rigid portions.
The printed circuits to be described hereinafter are generally very thin in cross section being constructed of multiple layers of film materials (conductive, insulative, and adhesive). In the interest of ease of drawing and understanding only, it should be readily recognized and understood by those skilled in the art that the drawing figures which accompany the written descriptions are not to scale.
Printed circuits combining rigid and flexible portions are known in the art and are popular in many applications such as automotive and aircraft. Such printed circuits allow madern printed circuit-mounted electronic components to be mounted and interconnected without the need for prior art wiring "harnesses" which were prone to damage, mis-wiring, and the like. As depicted in simplified form in FIG. 1 by way of example, such a combined printed circuit 10 may include several rigid portions 12 interconnected by flexible portions 14. Typically, the rigid portions 12 have components 16 and connectors 18 mounted thereon while the flexible portions 14 provide the interconnecting conductors 20 which replace the wires of the prior art wiring harnesses.
As is well known to those skilled in the art, printed circuits can be made by various deposition and etching processes. For example, one may start with an insulative substrate and deposit conductive paths and portions on one or both sides. In the alternative, one may start with an insulative substrate having a conductive layer on one or both sides and form the conductive paths and portions by removing un-needed portions of the conductive layer(s) using, for example, a chemical etching process through a mask. Such techniques are applicable to both rigid and flexible substrates so as to form rigid and flexible printed circuits. Since the manufacturing techniques are applicable to both rigid and flexible printed circuits, combined rigid and flexible printed circuits can be formed in a single process using a common technique.
While the first printed circuits were typically a single layer of substrate having conductive portions formed on one or both sides, many printed circuits employed now comprise several layers with the individual layers being adhesively bonded together into a unitary structure. To provide inter-layer electrical connections, aligned holes through the layers (known as vias) are internally plated with a conductive material. Under ideal conditions, the foregoing structure and general method of manufacture presents no problems. Under actual manufacturing conditions, however, various problems present themselves. The result is a diminishing of the yield of the manufacturing process; that is , the various problems to be described shortly result in defects in the resultant printed circuits which make them unreliable and, therefore, unusable. As those skilled in the art are well aware, process yield is a most important factor in electronics manufacturing. Pricing and competitiveness (as well as product quality and reputation) depend on high yields of reliable components and products produced therefrom. Thus, it is of vital importance that the manufacturing processes used to make such multi-layer, combined rigid and flexible printed circuits result in high yields and constantly reliable parts.
Typical prior art approaches to the construction and manufacturing of combined rigid and flexible printed circuits can be seen with reference to, for example, U.S. Pat. No. 4,687,695 (Hamby) or 4,800,461 (Dixon et al.). A typical prior art approach and its associated problems is depicted in simplified form in FIGS. 2 and 3. As with the printed circuit 10 of FIG. 1, there is a multi-layer printed circuit 10' having rigid portions 12 and a flexible portion 14. At the core or center of the circuit 10' there is a flexible substrate 22 having first conductors 24 formed on the outer surface thereof according to any of the many techniques known to those skilled in the art. The substrate 22 and first conductors 24 are protected on both sides by a flexible overcoating material 26 which adhesively bonds thereto. The rigid portions 12 are created by attaching a rigid substrate 28 over the overcoating material 26 employing a flexible adhesive material 30. The rigid substrate 28, of course, also includes second conductors 32 formed thereon as necessary.
While the rigid substrates 28 with the second conductors 32 pre-formed thereon could be adhesively attached to overcoating material 26, for registration reasons, the creation of the plated vias, etc. it is more typical to attach the rigid substrate 28 to the overcoating material 26 using aligning holes through the various layers (not shown) and to then form the second conductors 32 and the vias (shown ghosted as 34) in separate manufacturing steps. It is this approach which causes the manufacturing problems leading to reduced yield and reliability mentioned earlier herein. Because of the heat used in testing processes, there is considerable difference in the thermal expansion which occurs in the adhesive material 30 and the various other materials. As a result, voids are created in the adhesive material 30 and cracking may occur in the conductive plating material of the vias 34. This can cause patent defects which decrease the yield or, more serious, latent defects which can cause the final product to fail or malfunction suddenly and unexpectedly at a later time.
Another problem in the prior art is adverse chemical reactions during the manufacturing process. Typically, windows 36 are formed in the rigid substrates 28 in areas which are to be flexible in the final printed circuit 10'. To prevent the chemicals employed in formimg the vias 34 and/or second conductors 32 from damaging the underlying flexible portion 14, a blocking piece 38 is located in each window 36. A blocking piece 38 can be located in the window 36 as shown on the upper layer of FIG. 2; but, in such case, will be relatively ineffective in blocking the chemicals, which can simply flow around the edges and under the blocking piece 38. The blocking piece may also move out of registration. More typically, therefore, in such a prior art approach to manufacturing, the blocking piece 38 is held in place by adhesive 40. This aids, to aome degree, the chemical leakage problem--but doesn't always and completely solve it. On the other hand, it can introduce a new problem when removed after manufacture as depicted in FIG. 3.
A solution to this latter problem is disclosed in copending application Ser. No. 07/209,826, filed 6.22.88, which is assigned to the common assignee of this application. It is also depicted in simplified form in FIG. 4 hereof. For ease of understanding, elements of the printed circuit 10" of FIG. 4 which are common to the printed circuit 10' FIGS. 2 and 3 are designated with common numbers. Thus, there is once again a flexible substrate 22 having first conductors 24 formed on the outer surface thereof at the core or center of the circuit 10". The substrate 22 and first conductors 24 are protected on both sides by a flexible overcoating material 26 adhesively bonded thereto. The rigid portions 12 comprise a rigid substrate 28 including second conductors 32 formed thereon as necessary. At that point, the construction of the printed circuit 10" changes to implement the novel aspects of that co-pending application. As depicted in the bottom layer of the printed circuit 10" of FIG. 4, a protective layer 42 is placed over the entirety of the overcoating material 26--but, only adhesively attached (as with adhesive 44) over those portions thereof which are to be part of the rigid portions 12. In other words, in the "window" areas 36 the protective layer 42 simply covers the overcoating material 26, but is not adhesively attached thereto. The rigid substrate 28 is then attached to the protective layer 42 with adhesive material 30. The blocking pieces 38 or portions 38 are integral with but partially separated from the remainder of substrate 28 so that they may be readily removed after manufacture. When the manufacturing process is complete and the chemicals have been washed from the completed printed circuit 10", the blocking pieces 38 and the portions of the protective layer 42 within the window 36 are removed as depicted in the top layer of the printed circuit 10" of FIG. 4. No damage occurs to the overcoating material 26 and the portions under it when the protective layer 42 within the window 36 is removed since it is not adhesively attached to the overcoating material 26.
As those skilled in the art will readily recognize and appreciate, the novel approach of the above-described co-pending application solves the problem of the blocking pieces 38 and eliminates the possibility of chemical damage of the prior art over which it is an improvement; however, it does not address the above-described problem of voids being created in the adhesive layers and/or the problem of cracking of the plating within the vias from thermal expansion differences.
Wherefore, it is the object of this invention to provide a construction for printed circuits combining rigid and flexible portions and a method of manufacture thereof which eliminates not only the possibility of chemical damage but also both the problem of voids being created in the adhesive layers and the problem of cracking of the plating within the vias from thermal expansion differences.
Other objects and benefits of the invention will become apparent to those skilled in the art from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.